The present invention relates to converting a naphtha hydrocarbon feed to produce hydrocarbon compounds containing light olefins and aromatics. In particular, the present invention relates to conversion of a C4+ naphtha feed and includes the use of an intermediate pore zeolite catalyst.
Gasoline is the traditional high value product of fluid catalytic cracking (FCC). Currently however, the demand for ethylene and propylene is growing faster than gasoline and the olefins have higher value per pound than does gasoline. In conventional fluid catalytic cracking, typically less than 2 wt. % ethylene in dry gas is obtained, and it is used as fuel gas. The propylene yield is typically 3-6 wt. %.
Catalytic cracking operations are commercially employed in the petroleum refining industry to produce useful products, such as high quality gasoline and fuel oils from hydrocarbonxe2x80x94containing feeds. The endothermic catalytic cracking of hydrocarbons is most commonly practiced using Fluid Catalytic Cracking (FCC) and moving bed catalytic cracking, such as Thermofor Catalytic Cracking (TCC). In FCC, a cyclic mode is utilized and catalyst circulates between a cracking reactor and a catalyst regenerator. In the cracking reactor, hydrocarbon feedstock is contacted with hot, active, solid particulate catalyst without added hydrogen, for example at pressures up to 50 psig (4.5 bar) and temperatures of about 425xc2x0 C. to 600xc2x0 C. As the hydrocarbon feed is cracked to form more valuable products, carbonaceous residue known as coke is deposited on the catalyst, thereby deactivating the catalyst. The cracked products are separated from the coked catalyst, the coked catalyst is stripped of volatiles, usually with steam in a catalyst stripper, and the catalyst is then regenerated. Decoking restores catalyst activity while the burning of the coke heats the catalyst. The heated, regenerated catalyst is recycled to the cracking reactor to crack more feed.
In order to produce higher yields of light olefins, e.g. propylene and butylene, in conventional FCC reactors, the trend has been to dilute phase riser cracking with a brief hydrocarbon feed residence time of one to ten seconds. In this method, a small amount of diluent, e.g., steam up to 5 wt. % of the feed, is often added to the feed at the bottom of the riser. Dense bed or moving bed cracking can also be used with a hydrocarbon residence time of about 10 to 60 seconds. The FCC process generally uses conventional cracking catalyst which includes large pore zeolite such as USY or REY. A minor amount of ZSM-5 has also been used as an additive to increase FCC gasoline octane. Commercial units are believed to operate with less than 10 wt. % additive, usually considerably less.
U.S. Pat. No. 5,389,232 to Adewuyi et al. describes an FCC process in which the catalyst contains both conventional large pore cracking catalyst and a ZSM-5 additive. The patent indicates that the riser is quenched with light cycle oil downstream of the base to lower the temperature in the riser, since high temperatures degrade the effectiveness of ZSM-5. Although the ZSM-5 and the quench increase the production of C3/C4 light olefins, there is no appreciable ethylene product.
U.S. Pat. No. 5,456,821 to Absil et al. describes catalytic cracking over a catalyst composition which includes a large pore molecular sieve and an additive of ZSM-5 in an inorganic oxide matrix. The patent teaches that an active matrix material enhances the conversion. The cracking products included gasoline, and C3 and C4 olefins but no appreciable ethylene.
European Patent Specifications 490,435-B and 372,632-B and European Patent Application 385,538-A describe processes for converting hydrocarbonaceous feedstocks to olefins and gasoline using fixed or moving beds. The catalysts included ZSM-5 in a matrix which included a large proportion of alumina.
Although modifying conventional FCC processes to increase light olefin production can increase the yield of ethylene and especially propylene, increasing petrochemical propylene recovery from refinery FCC""s competes with alkylation demand. Moreover, the addition of ZSM-5 to the FCC reactor to increase propylene production, not only lowers gasoline yields, but may affect gasoline quality. Thus, many of the proposed modifications to a conventional FCC process will have undesirable effects on motor fuel quality and supply, resulting in the need for additional processing or blending to achieve acceptable motor fuel quality.
Thus, it would be advantageous to upgrade low value refinery streams to ethylene and propylene, while producing high quality motor fuels via conventional FCC processes.
In that regard, other types of processes have been developed for producing olefins from paraffinic feeds such as intermediate distillate, raffinate, naphtha and naphthenes, with olefin production directly or indirectly, as described, for example, in U.S. Pat. No. 4,502,945 to Olbrich et al., U.S. Pat. No. 4,918,256 to Nemet-Mavrodin, U.S. Pat. No. 5,171,921 to Gaffney et al., U.S. Pat. No. 5,292,976 to Dessau et al., and EP 347,003-B. The paraffinic feeds do not contain any significant amount of aromatics. These processes differ not only in feed, but in process conditions, variously including, for example, a requirement for addition of hydrogen (hydrocracking), use of high space velocities, accepting low conversions per pass and use of alumina or other active binders for the catalysts. In addition, little coke is produced on the catalyst so that fuel gas must be burned to generate heat for the endothermic reaction. Furthermore, there is little or no aromatic gasoline range product.
U.S. Pat. No. 4,980,053 to Li et al. describes catalytic cracking (deep catalytic cracking) of a wide range of hydrocarbon feedstocks. Catalysts include pentasil shaped molecular sieves and Y zeolites. Although the composition of the pentasil shape selective molecular sieve (CHP) is not particularly described, a table at column 3 indicates that the pentasil catalyst contains a high proportion of alumina, i.e., 50% alumina, presumably as a matrix. Deep Catalytic Cracking (DCC) is discussed by L. Chapin et al., xe2x80x9cDeep Catalytic Cracking Maximizes Olefin Productionxe2x80x9d, as presented at the 1994 National Petroleum Refiners Association Meeting. Using a catalyst of unspecified composition, the process produces light olefins of C3-C5 from heavy feedstocks. See also, Fu et al., Oil and Gas Journal, Jan. 12, 1998, pp 49-53.
It is an object of the invention to provide a catalytic conversion process with increased yield of C2 and C3 olefins and relatively low yield of C1 and C2 paraffins, while also producing useful aromatics.
The invention includes a process for converting a C4+ naphtha hydrocarbon feed to hydrocarbon products containing light olefins and aromatics by contacting the feed with a catalyst which comprises zeolite ZSM-5 and/or ZSM-11, having an initial silica/alumina ratio below about 70, a substantially inert binder and phosphorus. The contacting is under conditions to produce light olefin product comprising ethylene and propylene and aromatics comprising toluene and xylene.
The zeolite is bound with a substantially inert matrix material. The substantially inert matrix material comprises silica, clay or mixtures thereof. By substantially inert is meant that the matrix preferably includes less than about
20 wt. % active matrix material, more preferably less than 10 wt. % active material based on catalyst composition. Active matrix materials are those which have catalytic activity with non-selective cracking and hydrogen transfer. The presence of active matrix material is minimized in the invention. The most commonly used active matrix material is active alumina. The catalyst composition used in the invention preferably includes less than 20 wt. % alumina, more preferably less than 10 wt. % alumina, or essentially no active alumina. However, non-acidic forms of alumina such as alpha alumina can be used in these small amounts in the matrix. A small amount of alumina may be used to confer sufficient xe2x80x9chardnessxe2x80x9d in the catalyst particles for resistance to attrition and high temperatures but without introducing any appreciable non-selective cracking or hydrogen transfer.
The conditions minimize hydrogen transfer and it is preferred to avoid hydrogen addition, hydroprocessing and the use of other catalyst components which would introduce excess hydrogen transfer activity. It has also been discovered that the process can be conducted at generally higher temperatures than conventional, commercially practiced fluid catalytic cracking. High temperature operation also increases the rate of conversion to desired products relative to hydrogen transfer. Catalytic conversion conditions include a temperature from about 950xc2x0 F. (510xc2x0 C.) to about 1300xc2x0 F. (704xc2x0 C.), a hydrocarbon partial pressure from about 2 to about 115 psia (0.1-8 bar), a total system pressure of about 1-10 atmospheres, a catalyst/oil ratio from about 0.01 to about 30, and a WHSV from about 1 to about 20 hrxe2x88x921. In order to provide heat for the endothermic reaction, the catalyst is preferably hot, regenerated catalyst such as may be obtained by continuously circulating from the regenerator.
The products of the catalytic conversion process include light olefins and aromatics, and less than about 10 wt %, preferably less than about 8 wt % and more preferably less than about 6 wt % dry gas (methane and ethane). The product light olefins can include ethylene plus propylene in an amount of at least 20 wt. % based on total product; or at least 25 wt. %, and even up to 30 wt. % or more ethylene plus propylene. The product light olefins contain a significant amount of ethylene relative to propylene, with an ethylene/propylene weight ratio greater than about 0.39, preferably greater than about 0.6.
The process can be practiced in a fluid bed reactor, fixed bed reactor, multiple-fixed bed reactor (e.g. a swing reactor), batch reactor, a fluid catalytic cracking (FCC) reactor or a moving bed catalytic cracking reactor such as Thermafor Catalytic Cracking (TCC). A C4+ naphtha feed is catalytically converted in a catalytic reactor (e.g. an FCC reactor) operating under reaction conditions by contacting the feed with a catalyst containing ZSM-5 and/or ZSM-11, phosphorus and a substantially inert matrix, the contacting producing a product effluent which includes light olefins and aromatics. During the reaction, coke is formed on the catalyst. The product effluent and the catalyst containing coke are separated from each other. The effluent is recovered and the catalyst containing coke is regenerated by contact with oxygen-containing gas to burn off the coke and produce hot, regenerated catalyst and to produce heat for the endothermic reaction. The hot, regenerated catalyst is recycled to the catalytic reactor.
Advantageously, the process produces valuable light olefins and aromatic products useful as petrochemical feedstocks, with a relatively high ethylene to propylene ratio and without producing significant amounts of methane or ethane.